Energy and environmental issues have become urgent in recent years, and various measures have been suggested to use energy effectively.
As one of the measures, energy saving in household electrical appliances including refrigerators as consumer products is drawing attention. To achieve energy saving in refrigerators, it is necessary to use cold heat effectively and to improve the insulation of the insulating box which forms a refrigerator housing. For this purpose, it is effective that the insulating box is made of an insulation material with high insulation performance. As such high-performance insulation material, vacuum heat insulation materials containing glass fiber as a core material are being used these years.
A vacuum heat insulation material is formed by inserting a core material made of a porous body into a covering material and vacuum-sealing the covering material. Therefore, the covering material is required to have both high gas-barrier properties to maintain the internal pressure for a long time and a durability to prevent the formation of pinholes while being handled.
For this reason, a packaging material is typically composed of an outermost layer, an intermediate layer, and an innermost layer made of the following materials. The outermost layer is a protective film such as a polyethylene terephthalate film, the intermediate layer is an aluminum foil or a deposited film with excellent gas-barrier properties, and the innermost layer is an olefin-based sealant film with excellent heat-sealing properties.
Japanese Patent No. 2568485, for example, describes an insulator package as a vacuum heat insulation material with excellent resistance to pinholes that can be caused when the covering material is damaged. The insulator package includes a hermetically sealed container composed of the following four films: a nylon film, a polyethylene terephthalate film, a gas barrier layer, and a polyethylene film which are laminated in this order from outside to inside.
Since the nylon film laminated as the outermost layer of the covering material excels in scratch resistance and pinhole resistance, this conventional structure is effective to prevent the pinholes that can be caused by external friction or foreign bodies. However, this resistance is not sufficient to the pinholes that can be caused from the innermost sealant layer side by foreign bodies entered during vacuum packaging.
Japanese Patent Unexamined Publication No. H11-79234 describes a packaging material suitable to vacuum packaging. This packaging material includes an expanded plastic film and a sealant layer which are stacked on a 10 to 70 g/m2 thick paper base with joining layers disposed therebetween, the expanded plastic film being a metal oxide deposited layer. This structure can form the corners of a material to be vacuum-packaged in a smooth manner and also prevent a reduction in barrier properties such as moisture barrier properties after the material is vacuum-packaged.
This conventional structure is effective to prevent the pinholes that can be caused by impact or friction from outside, but not to the pinholes that can be caused by foreign bodies entered during vacuum packaging, or caused from inside the innermost layer in the case of some types of materials to be packaged.
Japanese Patent Unexamined Publication No. H09-317986 discloses an attempt to improve the puncture resistance of a plastic layer so as to provide a vacuum heat insulation material with excellent resistance to the pinholes that can be caused by the piercing of foreign bodies.
In this conventional vacuum heat insulation material, the films composing the covering material are strongly laminated to each other with high peel strength. A film can be broken when subjected to the piercing stress of a sharp-point foreign body because the film is compressed due to stress concentration. When compressed, an object increases in length in the direction perpendicular to the compressive stress. When broken, the object is subjected to a sudden shear stress in the direction perpendicular to the compressive stress. The force caused in the perpendicular direction by the compression acts as a tensile stress on the films laminated with the adhesive, thereby breaking these films concurrently. This results in a through-pinhole even when a foreign body is shorter in length than the thickness of the covering material.
Japanese Patent Unexamined Publication No. 2003-340972 describes the provision of a layer with low peel strength so as to prevent a plastic laminate film from being broken or having pinholes. This conventional structure discloses an impact resistant packaging material used for a packaging bag for storing frozen food or frozen materials with sharp edges. The impact resistant packaging material prevents the packaging bag from being broken by vibration during transportation in the low-temperature distribution or having pinholes by drop impact caused by inappropriate handling.
FIG. 12 is a sectional view of a conventional impact resistant packaging material. In FIG. 12, impact resistant packaging material 96 consists of the following: base material 91 formed of a synthetic resin layer; print ink layer 95 formed on a surface of base material 91 excluding sealing section 97 on the periphery of base material 91; impact resistant resin layer 92 formed on print ink layer 95 in such a manner as to cover the entire surface of base material 91; joining layer 93 formed thereon, and sealant layer 94 formed further thereon. Another method for producing impact resistant packaging material 96 is as follows. Print ink layer 95 is formed on a surface of base material 91 excluding sealing section 97 on the periphery of base material 91. Impact resistant resin layer 92 is formed exclusively on print ink layer 95. Sealant layer 94 is formed on impact resistant resin layer 92 via joining layer 93, which is formed on the entire surface of base material 91.
Impact resistant packaging material 96 with the conventional structure can be prevented from having through-pinholes when subjected to an external impact such as dropping with ice or other products inside. The prevention can be achieved by forcing print ink layer 95 with low peel strength to be peeled when base material 91 is broken.
The laminate film composing this conventional impact resistant packaging material, however, has the following problems when used as the covering material of a vacuum heat insulation material. The laminate film is pressed against the core material by the atmospheric pressure, so that the excessive parts of the laminate film that cannot tightly cover the core material are folded to form numerous wrinkles. As a result, in the laminate film including a print ink layer with low peel strength, the wrinkles formed on the print ink layer cause the film bases to be easily peeled from each other.
In this conventional packaging material, the main purpose of improving the pinhole resistance is to prevent the packaging materials from being torn by impact or being pierced by contents with a certain bulk such as frozen materials. Therefore, the improvement of the pinhole resistance is not sufficient for the pinholes that can damage the covering material of the vacuum heat insulation material, such as pinholes caused by glass shot, powder dust, and minute foreign bodies including glass fiber.
Unlike ordinary vacuum packaging materials, vacuum heat insulation materials are required to maintain high vacuum for a long time of ten years, thus needing a laminate film with high gas-barrier properties. Therefore, the conventional packaging material, in which insufficient gas barrier properties cause degradation in insulation performance with time, cannot be used as the covering material for vacuum heat insulation materials.